Electrical stimulation of a peripheral nerve (nerve bundle) is required by certain electro-neuroprostheses to replace or augment physiological body functions that have been compromised or rendered ineffective by disease or trauma. For example, U.S. Pat. Nos. 4,585,005; 4,607,639; and 4,739,764, issued to inventors named herein, disclose systems and methods designed for this purpose. Peripheral nerve electrodes basically consist of a metallic conducting material in the form of an electrode contact having an attached lead and an insulating material that supports the electrode contact.
The electrode contact is connected by its lead to a receiver, implanted on a patient, whereby selective stimulation of the nerve can be achieved in a conventional matter. The insulating material provides a substrate or carrier that functions to hold the electrode contact in a fixed position and further functions as a dielectric to confine electrical current so that it does not affect adjacent nerves, outside of the target area. The electrode contact is normally composed of a platinum-irridium alloy that is innocuous to living tissue, but yet delivers electrical current to the contact at an acceptable level to activate (stimulate) the target nerve. The insulating material composing the carrier is biocompatible, such as a Silastic wrapping or sheeting capable of providing the supporting and dielectric desiderata discussed above.
Various geometric forms of electrode contacts have been proposed, such as small circular or rectangular "buttons" and narrow foil or small wire configurations that are stripped of insulation adjacent to the point whereat a target nerve is positioned. The carrier is normally formed as a "cuff" that encircles the nerve or as a "spiral" that wraps around the nerve. The so-called cuff electrode is placed around the nerve to form a continuous slit between adjacent edges thereof. Sutures or staples are normally used to attach the two adjacent edges of the cuff electrode together.
Application of these sutures or staples to the cuff electrode requires a delicate surgical procedure after the cuff electrode is placed on the nerve. Unless the separation line between the opposing edges of the cuff electrode is properly sealed, the nerve is susceptible to herniation upon the growth of post-surgical tissue within the electrode lumen.
Another problem encountered with the use of the cuff electrode is the relative rigidity of the carrier to which the electrode contact is secured when the carrier is wrapped around a nerve. In particular, when the carrier is formed into a cylinder around the nerve, no space is left for accommodating tissue expansion when tissue grows between the electrode contact and the carrier whereby the nerve will assume a confined, constricted volume. Thus, in order to prevent such constriction, the cuff electrode must be formed to have a relatively larger diameter than the nerve to which it is attached to allow for such tissue growth. The use of an oversized cuff electrode obviously decreases the efficiency of the stimulus/electrode system.
In the case of the spiral electrode, the composite outer diameter thereof will normally increase to accommodate tissue growth. However, since the spirals of the electrode that wrap around the nerve are relatively narrow, the desired insulating properties of the spiral electrode will be absent. Therefore, non-targeted nerves, adjacent to the target nerve, will be stimulated by the leakage of current, particularly when relatively high current levels are used.